1. Technical Field
The present invention relates to an ultrasound measurement apparatus which has a function of specifying the position of a layer structure of a vascular wall on the basis of a reflected wave signal of an ultrasonic wave irradiated onto a blood vessel, or the like.
2. Related Art
In recent years, an ultrasound measurement apparatus is used to measure or diagnose arteriosclerosis of a blood vessel or a cardiovascular function. For example, in order to diagnose arteriosclerosis, it is necessary to measure the IMT (Intima Media Thickness) of a blood vessel. This is equal to measuring the distance between a lumen-intima interface and a media-adventitia interface of a vascular wall, and measurement precision of these interfaces is essential for appropriate diagnosis of the IMT.
In measuring the distance between the lumen-intima interface and the media-adventitia interface of the vascular wall, for example, an ultrasound probe having an array of a plurality of ultrasound transducers arranged therein is applied to the neck to irradiate an ultrasound beam. Then, a reflected wave signal (RF signal) applied to and reflected from the carotid artery is received by the ultrasound probe, and the reflected wave signal is processed to search a vascular wall.
While the ultrasonic signal is irregularly reflected in a living body, the ultrasonic signal tends to be comparatively strongly reflected at the interface of the lumen and the vascular wall of the blood vessel or the interface between the membranes of the vascular wall. For this reason, for example, a position at which the strength (amplitude) of the reflected wave signal is great is detected by differential processing, threshold value processing, or the like, thereby searching the position of the vascular wall.
However, the reflected wave of the lumen-intima interface has signal strength smaller than the signal strength of the reflected wave of the media-adventitia interface and is likely to be buried in noise. Specifically, an anterior wall (a vascular wall portion on a side close to the ultrasound probe) is likely to be affected by multiple reflection of the ultrasonic wave, and the contour is likely to be blurred. For this reason, it is difficult to stably measure the IMT of the anterior wall or a posterior wall (a vascular wall portion on a side separated from the ultrasound probe).
In connection with this, JP-A-11-318896 discloses a technique which translates the signal strength of the reflected wave to brightness and searches a brightness peak to automatically calculate the value of the IMT. Although a method which searches a brightness peak is difficult to detect the lumen-intima interface for the above-described reason, JP-A-11-318896 discloses a method which complements for this using regression curve correction or the average value of detected positions. However, in this method, when there are many positions which cannot be detected, IMT measurement precision may be degraded and an error to a true value may increase.
To begin with, since the thickness of the vascular wall, the wall composition, or the like differs for every individual, precision or credibility of detecting an interface of a layer structure of a vascular wall from a reflected wave signal of an ultrasonic wave from a living body differs depending on a living body to be measured. Accordingly, even if the IMT is automatically calculated using a known technique, there is a problem in that it is not possible to determine how much credible and stable is the value of the IMT.